The electric power steering apparatuses include a device of a so-called rack assist type wherein a driving force of an electric motor for steering assist is applied to a steer shaft (rack shaft) via a speed reduction mechanism and a ball screw mechanism thereby axially moving the steer shaft.
The above speed reduction mechanism includes: an input gear connected with an output shaft of the electric motor; and an output gear meshed with the input gear. The above motion converting mechanism includes a ball nut rotated integrally with the output gear, such as to convert the rotation of the output gear into axial movement of the steer shaft.
According to an electric power steering apparatus disclosed in Japanese Unexamined Patent Publication No. 2003-127879 published from Japanese Patent Office on May 8, 2003, for example, a pair of bearings for supporting the ball nut are disposed only on one side of the output gear while the pair of bearings disposed only on one side of the output gear are supported by a common split housing.
However, the ball nut is prone to displacement relative to the housing. In the event of the displacement of the ball nut, there occurs an error in relative positions between the output gear and the input gear. As a result, the input gear and the output gear are in poor meshing engagement, causing noises in the speed reduction mechanism.
According to an electric power steering apparatus disclosed in Japanese Unexamined Patent Publication No. 2002-274397 published from Japanese Patent Office on Sep. 25, 2002, on the other hand, a housing, which supports the steer shaft for axial slide movement, rotatably supports the ball nut via first and second bearings disposed on the opposite sides of the output gear. The above housing includes a first split housing for supporting the first bearing and a second split housing for supporting the second bearing.
Since the first and second bearings are retained by the first and second split housings respectively, the precisions of relative positions between the first and second bearings are affected by assembly precisions of the first split housing and the second split housing.
The error in the relative positions between the first and second bearings causes displacement of the ball nut and the output gear, so that an error occurs in the relative positions between the output gear and the input gear. Accordingly, the input gear and the output gear may be in poor meshing engagement, causing the noises in the speed reduction mechanism.
It is an object of the invention to provide an electric power steering apparatus adapted for positive prevention of the occurrence of noises.